1. Field of the Invention
The invention relates to a plain bearing material comprising a matrix material and a sliding material of at least one fluorothermoplastic and fillers. The invention also relates to the use of such plain bearing materials.
2. Description of Related Art
Bearing materials with plastics-based overlays are known as single-layer, two-layer or three-layer composite materials: solid plastics bearings, bearings with an outer metallic backing and directly applied or adhered plastics, other such with inner wire meshes, as well as three-layer bearings of backing metal, a sintered porous metal layer and a covering layer formed in the pores. All these bearings are generally used in areas in which the use of lubricants is impossible or undesirable. For this reason, they must provide these lubricants themselves when in operation.
Multilayer materials differ from solid plastics materials, for example by a negligible tendency towards cold flow under load, by substantially better heat conductivity and, in connection therewith, by markedly higher possible pv values. However, solid plastics materials may also be advantageous in certain cases, e.g. for reasons of cost.
Among three-layer materials, it is possible to distinguish further between those with overlays based on fluorothermoplastics, such as polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylene-propylene (FEP) etc., and those with overlays based on other plastics, such as polyetheretherketone (PEEK) for example. The latter two groups differ in their manner of operation: while, in the case of PTFE-based materials, the bronze intermediate layer is the "active" load-bearing component of the overlay and acts like a filler, the other plastics materials use it only as an anchoring means. If there is sufficient affinity to the metal backing, they permit the production of true two-layer materials, but may also be applied with the aid of an adhesive. On the active overlay itself the thermoset or thermoplastics material then assumes the supporting role of the bronze. Bearing materials of filled fluorothermoplastic films adhered to metal or other such materials with wire meshes incorporated in plastics are also known, which may likewise be adhered to a metal backing.
For universal applicability and ease of production, the most advantageous materials are three-layer materials based on fluorothermoplastics such as PTFE, which also exhibit the highest performance and temperature-resistance. In the production process, homogeneous PTFE/filler pastes are produced by means of a plastics dispersion and the final composite material is produced by a concluding step comprising sintering of the PTFE subsequent to rolling thereof onto the backing material.
The most commonly used fillers for such materials are lead and molybdenum disulphide, these materials providing virtually equal performance levels. These fillers may also be used in the presence of lubricants.
In many cases, it would be desirable to be able to solve constructional problems by using maintenance-free, space-saving plain bearings with PTFE overlays. However, the upper load limit, which, at a pv value of 2 MPa m/s, lies within the average loading and speed range (0.5-100 MPA and 0.02-2 m/s), may restrict use of these plain bearings.
It is known from DE 41 06 001 A1 that plain bearing materials with better performance levels may be also produced by using PbO as the filler, but there is a growing stigma attached to the use of materials, such as lead, which are potentially damaging to health. Furthermore, such materials, which are of optimum suitability for lubricant-free use, are unsuited for example to use as guide bushings for shock absorber piston rods, because under these conditions they do not exhibit the necessary wear and cavitation resistance or else their coefficients of friction are unsatisfactorily high.
Bearing materials with overlays consisting only of PTFE and molybdenum disulphide have long been known and are currently some of the most frequently used lubricant-free bearing materials. Boron nitride, which is known to be a solid lubricant, is also repeatedly named as a possible filler for PTFE despite the fact that the lubricant properties then only become effective at temperatures of over 800.degree. C.
Thus, both molybdenum sulphide and boron nitride are already mentioned in DE-PS 11 32 710 as Examples in a list of solid lubricants. The simultaneous use of molybdenum disulphide and boron nitride in PTFE is not mentioned, however.